Microstructural Influences on the Development and Growth of Small Fatigue Cracks in the Near Threshold Regime

Orientation imaging microscopy (OIM) is being used to investigate the microstructural influence on small crack growth at the crack tip. Although grain and grain boundary orientations have been reported to influence small crack growth, the effect has been difficult to quantify. OIM uses electron backscattered diffraction (EBSD) patterns in a scanning electron microscope (SEM) to form a spatially resolved map of crystal orientation, providing information on intra- and inter-grain orientation relationships.

Mini-extended compact tension (ECT) specimens were machined from an 1100 aluminum cold rolled sheet with a 45° angle, machined notch tip. The specimens were sized to fit into a Philips XL30/FEG SEM equipped with an EDAX/TSL EBSD/OIM system. Initial images were recorded prior to cycling of the mini-ECT specimen. Fatigue testing was conducted at constant amplitude loading in the near-threshold regime where small cracks are considered to be on the order of the corresponding grain size. The constant amplitude load testing was periodically interrupted to obtain EBSD images. The ability to map an area 800 × 1300 μm in front of the machined notch tip was demonstrated. This study determined that the plastic deformation, which occurs in a zone in front of the crack tip, did not degrade the EBSD image quality. Use of this technique provides the capability to characterize local crystal orientation during deformation processes such as fatigue crack growth.